Migrate to Docusaurus (#329)

* docs: initial migration to docusaurus

* website: add custom font, update blurbs and icons

* website: update splash

* root: update links to docs

* flows: use .pbflow extension so docusaurus doesn't mangle the files

* e2e: workaround prospector

* Squashed commit of the following:

commit 1248585dca
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:46:53 2020 +0100

    e2e: attempt to fix prospector error again

commit 1319c480c4
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:41:35 2020 +0100

    ci: install previous python version for upgrade testing

* web: update accent colours and format

* website: format markdown files

* website: fix colours for text

* website: switch to temporary accent colour to improve readability

* flows: fix path for TestTransferDocs

* flows: fix formatting of tests

website/docs/expressions/reference/user-object.md

@@ -0,0 +1,29 @@
+---
+title: User Object
+---
+
+The User object has the following attributes:
+
+-   `username`: User's username.
+-   `email` User's email.
+-   `name` User's display name.
+-   `is_staff` Boolean field if user is staff.
+-   `is_active` Boolean field if user is active.
+-   `date_joined` Date user joined/was created.
+-   `password_change_date` Date password was last changed.
+-   `attributes` Dynamic attributes.
+-   `group_attributes` Merged attributes of all groups the user is member of and the user's own attributes.
+-   `pb_groups` This is a queryset of all the user's groups.
+
+    You can do additional filtering like `user.pb_groups.filter(name__startswith='test')`, see [here](https://docs.djangoproject.com/en/3.1/ref/models/querysets/#id4)
+
+    To get the name of all groups, you can do `[group.name for group in user.pb_groups.all()]`
+
+## Examples
+
+List all the User's group names:
+
+```python
+for group in user.pb_groups.all():
+    yield group.name
+```